Manufacturers have been developing processes for synthesizing acrylic acid for decades.
The process which today is the most widely exploited industrially uses a catalytic oxidation reaction of propylene in the presence of oxygen.
This reaction is generally conducted in the gas phase, and most often in two steps: the first step carries out the substantially quantitative oxidation of propylene to give an acrolein-rich mixture, then, during the second step, the selective oxidation of acrolein to give acrylic acid is carried out.
The reaction conditions of these two steps, carried out in two reactors in series or in a single reactor containing the two reaction steps in series, are different and require catalysts suitable for the reaction; it is not however necessary to isolate the intermediate acrolein during this two-step process.
The gas mixture resulting from the second step consists, apart from acrylic acid:                of impurities resulting from the first reaction step which have not reacted;        of light compounds that are noncondensable under the temperature and pressure conditions customarily used, unconverted in the first step or formed in the second step: nitrogen, unconverted oxygen, carbon monoxide and dioxide formed in a small amount by final oxidation or going round and round, by recycling, in the process;        of condensable light compounds unconverted in the first step or formed in the second step: water, unconverted acrolein, light aldehydes such as formaldehyde and acetaldehyde, formic acid, acetic acid or propionic acid;        of heavy compounds: furfuraldehyde, benzaldehyde, maleic acid and anhydride, benzoic acid, 2-butenoic acid, phenol, protoanemonin.        
The complexity of the gas mixture obtained in this process makes it necessary to carry out a set of operations for recovering the acrylic acid contained in this gaseous effluent and convert it to a grade of acrylic acid that is compatible with the final use thereof, for example the production of acrylic acid polymers, or the production of acrylic ester polymers.
The first step of this recovery/purification phase consists of an extraction of the acrylic acid by countercurrent absorption in a solvent, generally water supplied by an external source and/or originating from the process. The amounts of water and of gaseous reaction mixture are such that the weight content of acrylic acid in the crude aqueous solution produced is generally of the order of 40% to 80%.
Nevertheless, a very significant economic problem is faced, mainly due to the expensive energy needed for eliminating the water used as acrylic acid absorption solvent, in so far as the effective elimination of the water without excessive loss of (meth)acrylic acid is complicated by the existence of interactions (hydrogen bonds) between the two compounds.
Thus, this separation operation is generally carried out on the industrial scale by distillation with a third azeotropic solvent, which contributes to increasing the number of distillation columns and their associated energy costs. Moreover, the increase in the number of distillation columns leads to an additional cost linked to the supplementary consumption of polymerization inhibitors that must be introduced into each of said columns in order to purify the desired product and eliminate the by-products while preventing the problems of fouling of the equipment by polymerization of the monomer.
One alternative to this process that uses water as acrylic acid absorption solvent is to use a hydrophobic heavy solvent to extract the acrylic acid, but such a process does not simplify the acrylic acid purification process.
Recently, in order to overcome these various drawbacks, new “solvent-free” technologies for recovering/purifying acrylic acid have appeared, involving a reduced number of purification steps and eliminating the introduction of external organic solvent.
In the acrylic acid production process described in U.S. Pat. No. 7,151,194, the gaseous reaction mixture is sent to an absorption column and brought into contact with an aqueous absorption solution, in order to obtain an aqueous solution of acrylic acid, which is then distilled in the absence of azeotropic solvent. A stream of crude acrylic acid is obtained as bottoms or as a sidestream from the distillation column, which is then sent to a unit for purification by crystallization. One drawback of this process is that the introduction of external water as absorption solvent makes it difficult to eliminate the water at the top of the absorption column without a significant loss of acrylic acid, and to recover a quality of crude acrylic acid having a low concentration of water as a sidestream, when this process is carried out in a two-column configuration.
Patent EP 2 066 613 describes a process for recovering acrylic acid without using external water, or azeotropic solvent and that only uses two columns for purifying the cooled gaseous reaction mixture: a) a dehydration column, b) and a finishing column (or purification column) fed by a portion of the bottom stream from the dehydration column.
The dehydration column generally operates at atmospheric pressure or slightly above atmospheric pressure.
In the dehydration column, the gaseous stream distilled as an overhead stream is condensed and sent back in part to the dehydration column in the form of reflux in order to absorb the acrylic acid.
The finishing column generally operates at a pressure below atmospheric pressure, which makes it possible to operate at relatively low temperatures, in order to thus prevent the polymerization of the unsaturated products present, and to minimize the formation of heavy by-products.
In the finishing column, the overhead distillate comprising water and light by-products is condensed then recycled to the bottom of the first column, and a stream comprising acrylic acid enriched in heavy by-products is eliminated as bottoms in order to be used possibly for the production of acrylic esters.
A stream of purified acrylic acid corresponding to a technical grade is recovered by drawing off as a sidestream in liquid or vapour form. The technical acrylic acid obtained generally has a purity of greater than 98.5 wt % and contains less than 0.5 wt % of water.
In this process, a portion of the streams (from the bottom of the dehydration column or from the top of the finishing column) is advantageously sent back to the heating/reboiler devices of the dehydration column and/or used for cooling the gaseous reaction mixture, which makes it possible to optimize the energy requirements of the process. Despite the advantages that the process described in document EP 2 066 613 provides, there still remain drawbacks linked to the implementation thereof.
In particular, at the top of the finishing column which functions under vacuum, the condenser releases residual vapours comprising organic impurities that must be eliminated, for example by incineration, which is harmful for the environment.
There are many vacuum-generating systems available for reducing the operating pressure of the distillation columns (see for example Techniques de l'Ingénieur, Pompes à vide [Vacuum pumps], B4030, Oct. 11, 1983). Generally a distinction is made between “volumetric” pumps which generate the vacuum by using liquid seals (oils, organic products or water), such as for example vane pumps and liquid ring pumps, and “drive” pumps, in which it is the flow of a fluid that creates the vacuum (ejector pumps, steam jet pumps, etc.). The systems most commonly used involve water or steam jet ejectors or liquid ring pumps, mainly water ring pumps.
These systems are not suitable for reducing the operating pressure of the finishing column of a solvent-free (meth)acrylic acid purification process, such as that described in document EP 2 066 613.
Such systems, for example described in document U.S. Pat. No. 6,677,482 or 7,288,169, use steam and generate a large amount of aqueous effluents containing acrylic acid and organic impurities, which cannot be economically recycled to the purification loop formed by the dehydration column and the finishing column. Indeed, it has been observed that the recycling of too large an amount of water at the dehydration column leads to sizeable losses of acrylic acid at the top of this column, unless an oversized column is used but this leads to an expensive investment. These aqueous effluents therefore need to be sent to a water treatment plant or directly to a thermal oxidizer, thus giving rise on the one hand to a loss of high quality product and on the other hand to an environmentally harmful discharge. The effluents from these vacuum systems, rich in noncondensable compounds which are incinerated or sent directly to the atmosphere, release in the first case oxidation products and in the second case organic compounds that are environmental pollutants.
Therefore, there remains a need to eliminate and/or reduce the generation of aqueous discharges in a solvent-free recovery/purification process enabling the finishing column to operate under vacuum.
The inventors have now discovered that the use of a dry vacuum pump linked to the operation of the finishing column in a solvent-free acrylic acid recovery/purification process makes it possible to meet this need, with the economic and environmental advantages that result therefrom.
It has become apparent to the inventors that this invention could be applied to acrylic acid produced from sources other than propylene, to methacrylic acid and also to these acids derived from renewable raw materials, which are capable of posing the same purification problems.